1. Field of the Invention
The present invention relates to a focus control apparatus for a digital imaging device, and especially to a focus control apparatus applicable to both step motor and piezoelectric material to control a distance between an electronic imaging device and a lens unit.
2. Description of Related Art
With the rapid progress of digital electronic and semiconductor process, many conventional consumer products have already been digitalized. For example, digital imaging devices such as digital still cameras (DSC) and digital video cameras (DV) are becoming more and more mature and popular.
The digital imaging device uses electronic imaging device such as CCD (charge coupled device) or CMOS (complementary metal oxide semiconductor) sensor instead of conventional film to obtain images. Moreover, a focusing unit is also essential for the performance of digital imaging device.
The present digital imaging device generally uses a lens unit controlled by a step motor to control the focusing of image. The step motor, based on a digital signal, controls the distance between the lens unit and the electronic imaging device for adjusting focus. However, the step motor is bulky, slow and expensive.
Alternatively, piezoelectric material such as piezoelectric ceramic can be used to implement a piezoelectric focusing unit for a digital imaging device. An electrical signal is applied to deform the piezoelectric material, so the distance between the lens unit and the electronic imaging device can be quickly adjusted. The distance for a focusing adjustment is generally a minute value. Therefore, the piezoelectric focusing unit is feasible, small, fast and cheap.
However, the piezoelectric material generally has a hysteretic response curve between driving force (electrical signal) and resulting deformation. The focus control apparatus for piezoelectric material is complicated in comparison with the focus control apparatus for step motor. Therefore, it is difficult to control the focusing action for both piezoelectric focusing device and step motor in the same digital imaging device simultaneously.